The present invention re lates to a seismic data acquisition method and apparatus and in particular to a seismic data acquisition method and apparatus adapted for use in a land or transition zone environment th at provides for improved seismic sensor position determination, thereby reducing perturbation noise generated by inaccurate assignment of seismic sensor position coordinates to seismic data.
Seismic data is collected to remotely sense subsurface geologic conditions, particularly in connection with the exploration for and production of hydrocarbon reserves, such as oil, natural gas, or coal. To gather seismic data, acoustic sources such as explosives or vibrators are used to produce an acoustic pulse that is transmitted through the subsurface geologic formations. Changes in acoustic impedance between different geologic layers cause a portion of the acoustic energy to be reflected and returned toward the earth's surface. These reflected signals are received by seismic sensors and are processed to create images of the subsurface geology.
Seismic data is typically acquired on land using strings of seismic sensors that are known as geophones. The geophone string is laid over a certain area in a given pattern. This pattern could either be linear or it could be a spatial pattern covering a certain area (typically 50 by 50 meters) with its center of gravity located in a predetermined position. This group of seismic sensors is typically called a station and the center of gravity is referred to as a station location.
A seismic data acquisition system can control and acquire data from a large number of seismic stations. In a two dimensional (2D) application, the stations are laid in one line which could include 1000 stations covering up to 50 kilometers. In a three dimensional (3D) application, the stations are typically organized in several parallel lines. Each line in a 3D seismic survey is similar to the line of a 2D application, with a typical line being perhaps 5 kilometers in length and each line including on the order of 100 or so stations. Between five to twenty lines, for instance, could be active at any given time during a 3D survey and they will typically be connected to a single set of seismic data recording equipment. Adjacent lines of a 3D survey could be separated by between 300 and 1000 meters, for instance.
A large 3D system can control 3000 or more stations at any time. Within a station, the grouping of the data from individual geophones attenuate noises such as horizontally traveling waves (ground roll) and random noises. Random noises can be generated by wind, rain, scratching of the geophone case by vegetation, geophone cable oscillation, etc. The level of noise attenuation depends, in part, on the pattern used. The data from multiple stations will be processed later on to provide the seismic image of the subsurface geology.
In a typical land seismic acquisition process, a 3D seismic survey is first planned to account for the client's geophysical objectives and to attempt to minimize terrain constraints on the seismic survey. The 3D seismic survey plan will generally include a seismic sensor lay-out scheme (including identifying all of the planned seismic station locations and the sensor lay out pattern) and a seismic source deployment scheme.
A survey crew will then travel to the seismic survey location to determine the positions on the earth's surface where the stations will be laid. The survey crew typically plants a survey peg (a small flag) at the center of each station. This peg is characterized by a unique identifier, a peg index. The surveyors then generate a correspondence table between the peg index values and the station coordinate values.
Later, the lay-out crew attempts to install the geophone string in the proper pattern, while visually ensuring that the center of gravity of the pattern is at the peg. The geophone string is then connected to a cable “take-out” connector of a seismic data transmission cable. In most case, the peg index (or station reference) and the electronics address on the seismic data transmission cable are incremented in the same pattern. However this is not always possible.
In the real world, many problems can occur that can generate errors when the position coordinates of the seismic station locations are matched with the acquisition channels of the seismic data acquisition system. With complex lay-out in difficult terrain, some acquisition channels may not be used to allow the bypassing of obstacles. The number of skipped take-out connectors must be properly recorded by the lay-out crew and transmitted to the recording computer so that this information can be used to properly map the relationship between the station location and the electronic channel address. In 3D applications, the line definition based on the electronic acquisition channel does not always represent the seismic line configuration on the ground. In fact, a single physical line cable can be folded to represent multiple seismic data acquisition lines. With the complex network structures in new seismic data acquisition systems, various network layers can be used to physically communicate to a given acquisition channel: this allows a large flexibility for the lay-out operation (i.e. a large contour). In these cases, the electronic addressing scheme does not represent the seismic line lay-out.
A similar problem occurs if a cross-line or backbone cable is used to transmit seismic data from a seismic data cable to the recording equipment. If the take-out at which the backbone cable is joined to the seismic data cable is not properly recorded and accounted for, it is possible to improperly shift the coordinate values attributed to the seismic data acquired by seismic sensors recorded using the seismic data cable.
The mapping between the electronic address and seismic station reference is conventionally performed at the central seismic recorder using information from the survey team, the lay-out team and the network definition. This logic can easily be corrupted. Errors generate major perturbations in the seismic processing, and the image quality of the processed seismic data can thereby be drastically decreased.
The quality of the seismic image obtained from the seismic data is also affected by other geometry problems. The center of gravity of the seismic sensor group should be as close as possible to the survey peg; otherwise the seismic image will be blurred. Seismic data is typically processed using algorithms that rely on accurate information regarding the position coordinates of the seismic sensors used to acquire the seismic data. When the actual sensor position coordinates differ from the sensor position coordinates that are assigned to the seismic data, the processing algorithms will often fail to correctly perform the intended data processing operation, and may in fact introduce noise or processing artifacts into the processed seismic data. The non-optimum group pattern may also limit the performance of some types of processing intended to reduce noise (especially operations intended to reduce coherent noise such as ground roll noise).
Group patterns and the group center of gravity are not always within acceptable tolerances, as the lay-out of the geophones are based on visual judgments of distance and position performed by a seismic crew member. There is currently very little quality control performed regarding geophone positioning. Sensor positioning errors (i.e. associating incorrect seismic sensor position coordinates with seismic data) typically generate noise behind the signal.
In a theoretical study performed to predict the level of noise that could be caused by various perturbations in a uniform spatial sampling seismic data acquisition system, perturbation noise was calculated before and after seismic processing which provided the following signal to noise ratios:                10% amplitude perturbation on a single geophone≈30 dB        1 millisecond static on a single geophone≈20 dB        0.5 meter difference in position of a single geophone≈40 dB        5 meter difference in position of group center≈20 dB        
After processing (DM0-stack) the noise level is reduced independent of the kind of perturbation by about 14 dB, which is expected from coverage of 20 to 60 fold. Doubling the amount of perturbations nearly results in a 6 dB increase of the perturbation noise before and after processing. With this study, it is clear that small positioning errors generate clear perturbations in the expected signal to noise ratios of seismic data. Major noise has to be expected when substantially incorrect seismic sensor positions are assigned to seismic data. While this study looks at only one particular case, it clearly shows the relative importance of these types of seismic data acquisition errors.
There are two primary types seismic sensor positioning errors that need to be corrected or prevented during seismic data acquisition. First, inaccurate matching of survey information with the physical deployment of a particular geophone string (i.e. the electronic acquisition channel of the seismic data acquisition system) has to be suppressed. This is achieved in the inventive method and apparatus by remotely detecting survey peg information on a particular seismic data acquisition channel. This type of correction or cross-checking is typically performed before the seismic data is acquired by the seismic sensors.
Second, induced noise in the seismic image due to improper correspondence between the assumed and actual positions of the geophones at each station, such as differences between the actual and the planned positions of the center of gravity of the geophone string, has to be reduced. To reduce these differences, the inventive method and apparatus provides for airborne acoustic positioning to determine the actual 2D positions of the geophones after lay-out. As a basic quality control procedure, geophones outside the adequate positioning tolerances can be identified and their position can either be corrected for, or the data from these sensors can be ignored.
A more sophisticated approach is to reposition the center of gravity of the group during digital data grouping output, using adequate mathematical compensation schemes. The subsequent seismic processing sequence can thus be modified or adapted to compensate for deviations between the planned position and the actual position of the group center of gravity. Digital filtering for noise attenuation and some types of seismic processing procedures depend of the geometry of the seismic sensor group. Suitable adjustments can be made to these processes to compensate for deviations between the optimum or assumed positions and the actual deployed positions. For example, if the geophones of a particular group are laid in a bunched fashion due to terrain constraints, the digital filter control parameters can be modified, or the digital filtering process can be skipped altogether for the seismic data acquired by the geophones in this group.
It is therefore an object of the present invention to provide for an improved method and apparatus for seismic sensor position determination.
An advantage of the described embodiment of the present invention is that the seismic sensors can be installed with larger tolerances, insuring easier and faster lay-out.
Another advantage of the described embodiment of the present invention is that the data quality can be higher after adequately correcting for any deviations between the planned seismic sensor deployment and the actual seismic sensor deployment.
A further advantage of the described embodiment of the present invention is that the likelihood of inaccurately assigning seismic sensor position coordinates to seismic data can be significantly reduced.